Spitzer Space Telescope pinpoints elusive but violent starbursts

A major breakthrough in pinpointing some of the most primordial and violently star forming galaxies in the Universe has been made by a joint collaboration of UK and US astronomers using the Spitzer Space Telescope to resolve primordial galaxies initially detected by the James Clerk Maxwell telescope [JCMT]. UK astronomers from the University of Kent, The Royal Observatory Edinburgh and the University of Oxford teamed up with American cosmologists to finally identify these elusive galaxies. The work will be published in the Astrophysical Journal Supplement Spitzer Special Issue in September 2004.

Back in 1995, the UK’s SCUBA camera (Sub-millimetre Common User Bolometer Array) on the James Clerk Maxwell Telescope in Hawaii, which detects light with wavelengths just under a millimetre, began finding fuzzy traces of very distant, primordial galaxies. Some of these are either too distant or too dusty to be seen even by the Hubble Space Telescope. But SCUBA’s images on their own, and those of other similar cameras, are not fine enough: within the fuzzy SCUBA detections are sometimes many galaxies. So astronomers have spent enormous effort following up these SCUBA galaxies on other telescopes, particularly radio telescopes, to answer the question: which one is the primordial galaxy, and which ones are in the foreground? But even with the most sensitive radio telescope images ever made, only around half the SCUBA galaxies can be pinpointed unambiguously. Even worse, the radio telescopes miss all of the most distant and most primordial of SCUBA’s galaxies.

UK and US astronomers teamed up to combine Spitzer’s sharp images with SCUBA’s ability to find primordial galaxies. The team were stunned to find all the SCUBA galaxies in Spitzer’s field of view detected in only ten minutes with Spitzer. These breakthrough observations, described as a ‘watershed’ by the team, finally give astronomers a way of unambiguously pinpointing even the most distant of SCUBA’s galaxies. This could only be done by combining SCUBA with the Spitzer Space Telescope: SCUBA shows there is a primordial, violent starburst somewhere in the vicinity, which is then pinpointed by Spitzer.

At the same time, Spitzer solved another mystery about SCUBA galaxies. When Galileo first trained a telescope at the Milky Way, he was astonished to find the fuzzy light resolved into many individual stars. This is, in essence, what the team of astronomers have done with the diffuse extragalactic background light seen from all directions at a wavelength of about half a millimetre. By comparing the distinct Spitzer galaxies with the SCUBA data, the team discovered that they had identified the sources of this cosmic background for the first time. This background is caused by an important population of galaxies: most of the stars in the early Universe are created in these galaxies, and star formation is where everything comes from – including the material that makes planets like our own. Finding where this star formation happens tells us, in a sense, where we came from. Identifying most of these galaxies is a second coup for the joint UK/US team.

Dr Stephen Serjeant (University of Kent, UK) said, ‘Our Spitzer Space Telescope images picked our galaxies out astonishingly quickly, in only ten minutes, when the community has been pouring effort into detecting them. This really is pioneering work and a great triumph for the Spitzer Space Telescope and the UK’s SCUBA camera. To cap it all, at the same time we’ve found the galaxies that dominate the star formation in the early Universe. The Earth and everything on it is made from the dust created in stars like those – people, trees, beef burgers, the lot.’

Dr Rob Ivison (Royal Observatory Edinburgh, UK) said, ‘In 10 minutes, the Spitzer Space Telescope has managed to pinpoint the galaxies we have been chasing for 7 years. We can finally begin to sort the babies and teenagers of the galaxy world from the adults and senior citizens.’

Dr Hervé Dole (University of Arizona USA and IAS, Orsay, France) said, ‘These Spitzer observations were designed as the first joint survey using the MIPS and IRAC instruments on Spitzer, to assess the instrument sensitivities. As a matter of fact, it’s a great technological, operational and scientific success, overwhelming our wildest expectations. This demonstrates the amazing capabilities of Spitzer for studying galaxy evolution at high redshifts; no doubt that deeper and larger ongoing surveys will give even more exciting results!’

Dr Steve Willner (Harvard-Smithsonian Center for Astrophysics, USA) said, ‘We expected to detect one or a few of these galaxies, but I was stunned that we detected all of the ones we looked at. The new data finally tell us what these galaxies are all about. We’ve known all along that they had to be far away and rapidly turning all their gas into stars, but now we know their true distances and ages.’